Polymerisation initiators, polymerisable compositions, and uses thereof for bonding low surface energy substrates

ABSTRACT

Quaternary boron salts are used as initiators of polymerisation in adhesive compositions for bonding low surface energy substrates such as polyolefins. The quaternary boron salts are of the formula I 
                         
wherein R 1  is C 1 –C 10  alkyl,
         R 2 , R 3  and R 4 , which may be the same or different, are, C 1 –C 10  alkyl or C 3 –C 10  cycloalkyl, phenyl, or phenyl-substituted C 1 –C 10  alkyl or C 3 –C 10  cycloalkyl, provided that any two of R 1 –R 4  may optionally be part of a carbocyclic ring, and   M +  is a metal ion or a quaternary ammonium ion.       
     Particular examples of initiator compounds include sodium tetraethyl borate and lithium tetraethyl borate, lithium phenyl triethyl borate and tetramethylammonium phenyl triethyl borate.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to the application entitled “PolymerisationInitiators, Polymerisable Compositions, and Uses Thereof” filed on evendate herewith as International application No. PCT/IE01/00133 claimingpriority from European Patent Application No. 00650166.2 filed on Oct.23, 2000; the contents of the said International application areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to polymerisation initiators, polymerisablecompositions particularly acrylic adhesive compositions and uses thereoffor bonding low surface energy substrates such as polyolefins to eachother or to other substrates such as metals.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

It is well known that the bonding of polyolefin substrates and other lowsurface energy substrates causes particular difficulties. Attempts havebeen made to overcome this problem by extensive and expensive substratesurface preparation, for example by oxidation, plasma treatment, coronatreatment or flame treatment, or by priming the surface with a highsurface energy primer. However it is desired to develop adhesivecompositions which will bond low surface energy substrates without suchsurface preparation.

The chemistry of organic boron compounds has been studied in detail (see“Comprehensive Organic Chemistry” by Barton D. and Ollis W. D., Volume3, Edited by Jones D. N., Pergamon Press, 1979,Part 14). The use oforganoboranes such as the trialkylboranes including triethylborane andtributylborane for initiating and catalyzing the polymerisation of vinylmonomers is well known. However such organoborane compounds are known tobe flammable in air so that the compounds and compositions containingthem require special handling and the compositions have poor shelfstability (see for example U.S. Pat. No. 3,236,823 Jennes et al., andthe Background section of U.S. Pat. No. 5,935,711 Pocius et al., atcolumn 2).

Certain boron alkyl compounds and their use as initiators ofpolymerisation are described in a series of patents of Wolfgang Ritterassigned to Henkel KgaA, including U.S. Pat. Nos. 4,515,724, 4,638,092,4,638,498, 4,676,858 and 4,921,921 (hereinafter referred to as “theRitter patents”). However the adhesive systems developed from thesepatents require the manufacture of trialkyl boranes from long chainfatty acids.

A series of patents of Skoultchi or Skoultchi et al. disclose a two-partinitiator system for acrylic adhesive compositions in which the firstpart includes a stable organoborane amine complex and the second partincludes a destabilizer or activator such as an organic acid or analdehyde (U.S. Pat. Nos. 5,106,928; 5,143,884; 5,286,821; 5,310,835 and5,367,746).

Japanese patent publication No. S48-18928 describes a method foradhering polyolefin or vinyl polymers using an adhesive obtained byadding trialkylboron to a vinyl monomer or vinyl monomer and vinylpolymer. Examples of trialkylboron include triisopropylboron,tri-n-butylboron, tripropylboron and tri-tert-butylboron.

U.S. Pat. No., 3,275,611, Mottus et al. describes a process forpolymerising unsaturated monomers with a catalyst comprising anorganoboron compound, a peroxygen compound with an amine complexingagent for the boron compound. Use of the polymerisation products asadhesives is not discussed.

U.S. Pat. No. 5,539,070 of Zharov et al. assigned to Minnesota Miningand Manufacturing Company and a series of patents of Alphonsus V. Pociusor Pocius et al. also assigned to Minnesota Mining and ManufacturingCompany, including in particular U.S. Pat. Nos. 5,616,796, 5,621,143,5,681,910, 5,684,102, 5,686,544, 5,718,977, 5,795,657 and 5,935,711describe organoborane amine complexes which can be used in systems thatinitiate the polymerisation of acrylic monomers in compositions usefulfor bonding low surface energy plastics substrates such as polyethylene,polypropylene and polytetrafluoroethylene. PCT Publication No. WO99/64528 also of Minnesota Mining and Manufacturing Company describeslow odour polymerisable compositions comprising monomer blends andorganoborane amine complex initiators. However these systems require thepreparation of trialkyl borane amine complexes to achieve the desiredperformance and shelf stability. The manufacture of such complexes is anundesirably complicated process. Furthermore the presence of the aminesresults in cured adhesives that have a tendency to become yellow incolour on ageing.

In the unrelated field of photopolymerisation for colour imagingmaterials, it is common to use cationic dye—borate anion complexes asphotopolymerisation initiators (see for example U.S. Pat. Nos.4,772,530, 4,772,541 and 5,151,520 of Gottschalk et al., assigned to TheMead Corporation). As particularly described in U.S. Pat. No 4,772,530,the complexes can be represented by the general formula:

where D⁺ is a cationic dye; and R¹, R², R³ and R⁴ are independentlyselected from the group consisting of alkyl, aryl, alkaryl, allyl,aralkyl, alkenyl, alkynyl, alicyclic and saturated or unsaturatedheterocyclic groups. The borate anion is designed such that the borateradical generated from exposure to light and after electronic transferto the dye readily dissociates with the formation of a radical asfollows:

It is stated that particularly preferred anions are triphenylbutylborateand trianisylbutylborate anions because they readily dissociate totriphenylborane or trianisylborane and a butyl radical. On the otherhand it is stated that tetrabutylborate anion does not work wellpresumably because the tetrabutylborate radical is not stable and itreadily accepts an electron back from the dye in a back electrontransfer and does not dissociate efficiently. Likewise,tetraphenylborate anion is very poor because the phenyl radical is noteasily formed.

The photopolymerisable compositions of the above-mentioned patentsassigned to The Mead Corporation are used in imaging materials, notadhesive compositions.

U.S. Pat. No. 4,950,581 Koike et al, assigned to Fuji Photo Film Co.Ltd, describes a photopolymerisation initiator for use in the field ofimaging, comprising a combination of (a) an organic compound representedby the formula:

wherein R¹, R², R³, and R⁴, which may be the same or different, eachrepresents a substituted or unsubstituted alkyl group, a substituted orunsubstituted aryl group, a substituted or unsubstituted alkenyl group,a substituted or unsubstituted alkynyl group, or a substituted orunsubstituted heterocyclic group, and at least two of said R¹, R², R³,and R⁴ may combine to form a cyclic structure, with the proviso that atleast R¹, R², R³, and R⁴ is an alkyl group, and wherein Z⁺ represents analkali metal cation or a quaternary ammonium cation, and (b) an organicdye having no counter anion.

Examples of such organic boron compounds include tetraethylammoniumtriphenylbutyl borate, tetramethylammonium triphenylbutyl borate,tetra-n-butylammonium tri-4-methoxyphenyl butyl borate,tetra-n-butylammonium triphenyl butyl borate, tetra-n-butylammoniumtetra-n-butyl borate, sodium triphenyl-n-butylborate,tetramethylammonium triphenylbenzylborate.

Suitable organic dyes having no pair anion for the photopolymerisationinitiator include merocyanine series dyes, coumarin series dyes, andxanthene or thioxanthene dyes. The Koike et al. patent is not concernedwith adhesive compositions.

U.S. Pat. No. 6,110,987 Kamata et al. assigned to Showa Denko K. K.describes a photocurable composition comprising an ultraviolet radicalpolymerisation initiator, a cationic dye with absorbtions in the visiblelight region, a quaternary boron salt and optionally a polymerisationaccelerator. The quaternary boron salt sensitizer is represented by thegeneral formula

wherein R₁, R₂, R₃, and R₄ each independently represent an alkyl groupwhich may have a substituent, an aryl group which may have asubstituent, an allyl group which may have a substituent, an aralkylgroup which may have a substituent, an alkenyl group which may have asubstituent, an alkynyl group which may have a substituent, a silylgroup which may have a substituent, a heterocyclic group or a halogenatom, and Z⁺ represents a quaternary ammonium cation, quaternarypyridinium cation, quarternary quinolinium cation, phosphonium cation,sulfonium cation, oxosulfonium cation, iodonium cation or metal cation,and mixing these with a compound which has a radical polymerizableunsaturated group.

Examples of the cation portion mentioned include tetramethylammonium,tetraethylammonium, tetra-n-butylammonium, tetraoctylammonium, lithiumcation and sodium cation. In the working examples, the cation portionsare all tetra-n-butylammonium, and the anion portions aren-butyltriphenyl borate, n-butyltri(4-t-butylphenyl) borate,n-butyltri(2-tolyl) borate, n-butyltri(4-tolyl) borate,n-butyltri(4-fluoro-2 methylphenyl) borate.

In the above described photopolymerizable compositions, cationic dyes orother organic dyes are used to absorb light and to facilitate transferof energy or electrons. The distinction between triorganylboranes andorganoborate salts is well illustrated in “Comprehensive OrganicChemistry” by Barton D. & Ollis W. D. (Ed. D Neville Jones) cited above,Volume 3, in which separate chapters, Chapters 14.3 and 14.4, aredevoted to them. The photopolymerisable compositions of theabove-mentioned patents are not intended for use as adhesives, sealantsand the like. Photopolymerisation systems operate by absorption of lightenergy, and the characteristic feature of such systems is that they havelight-absorbing ability. It is not predictable whetherphotopolymerisation initiators will be suitable for other cure systemsor for bonding substrates having particular surface energy conditions.In particular, the above-mentioned patents concerningphotopolymerisation initiators do not provide any disclosure or teachingrelating to the bonding of low surface energy substrates.

U.S. Pat. No. 6,171,700 Sugita et al. assigned to Showa Denko K. K. andShowa Highpolymer Ltd. describes a curable composite material comprising

-   (A) a fibre reinforcing material eg carbon cloth, aramid fibre    cloth, or continuous glass mat, and/or fibre filler eg. glass    roving,-   (B) a polymerizable unsaturated compound,-   (C) a polymerisation initiator comprising    -   (a) an organic boron compound similar to that described in the        above-mentioned U.S. Pat. No. 6,110,987,    -   (b) an acidic compound, and    -   (c) a hexaarylbiimidazole.

The acidic compound may be a latent acid-generating agent. In theworking examples a light/heat induced latent acid generator (sulfoniumcompound) is used. The process for curing the composite materialcomposition in the working examples comprises irradiating it. Thispatent is concerned with particular problems of photocuring compositionscontaining fibre reinforcing material and/or filler, particularly whenthe composite material composition is relatively thick or is aconstruction that inhibits light permeation. It does not contain anyteaching relevant to the bonding of low surface energy substrates.

There is a need for commercially acceptable compositions for bonding lowsurface energy substrates such as polyolefins and for end users to havea variety of such compositions which achieve that result throughdifferent technical strategies. Despite the work of many researchers inthis field, there is a need for polymerisation initiators which reducethe problems described above and which provide alternative systems tothose available hitherto. It is desirable to provide initiators whichare commercially available compounds and/or which are relatively easy tohandle compared to compounds of the prior art.

SUMMARY OF THE INVENTION

This invention relates to the use of a compound of the formula I:

wherein R¹ is C₁–C₁₀ alkyl,

-   -   R², R³ and R⁴, which may be the same or different, are C₁–C₁₀        alkyl or C₃–C₁₀ cycloalkyl, phenyl, or phenyl-substituted C₁–C₁₀        alkyl or C₃–C₁₀ cycloalkyl, provided that any two of R¹–R⁴ may        optionally be part of a carbocyclic ring, and    -   M⁺ is a metal ion, or a quaternary ammonium cation,        as an initiator of polymerisation in an adhesive composition for        bonding a low surface energy substrate.

The applicants are not aware of any disclosure or teaching of the use ofionic compounds of this kind as polymerisation initiators in adhesivecompositions for bonding low surface energy substrates.

The initiator compounds of formula I are quaternary boron salts.According to one aspect of the invention, M⁺ is a metal ion. If M⁺ is aquaternary ammonium cation, it may suitably be of the formula II:

wherein R⁵–R⁸, which may be the same or different, are C₁–C₁₀ alkyl,C₁–C₁₀ alkenyl, C₃–C₁₀ cycloalkyl, aryl, C₁–C₁₀alklylaryl,arylC₁–C₁₀alkyl or arylC₁–C₁₀ cycloalkyl, provided that any two of R⁵–R⁸may be part of an optionally unsaturated carbocyclic ring. Aryl issuitably optionally substituted phenyl, wherein the phenyl ring may besubstituted, for example with C₁–C₁₀ alkyl, particularly C₁–C₆ alkyl, orhalo, particularly Cl, Br or F. Examples of a quaternary ammonium cationinclude a tetra C₁–C₁₀ alkylammonium cation, particularly a tetra C₁–C₅alkylammonium cation, for example tetramethylammonium,tetraethylammonium or tetra-n-butylammonium, or a triC₁–C₁₀alkylarylammonium cation wherein aryl is phenyl, substituted phenyl(with phenyl being substituted as above), or phenyl-substituted C₁–C₁₀alkyl or C₃–C₁₀ cycloalkyl.

An alkyl group may suitably have 1–6 carbon atoms, for example 1–4carbon atoms. An alkyl group may be straight-chain or branched. Acarbocyclic ring may be bridged by the boron atom in formula I.

The initiator compounds used in the present invention are commerciallyavailable or are readily prepared by available methods. One exemplarycompound, sodium tetraethyl borate is available in solid form, thusenabling the formulation of solvent-free adhesives, which is asignificant advantage for environmental and labelling reasons, as wellas reducing potential flammability. The initiator compounds of theinvention do not require the use of amines which have been used in thepast to form covalent complexes with organoboranes. Once theorganoborane of such complexes has reacted in a polymerisablecomposition, the amine can cause yellowing of a cured composition afterageing. A reduction or elimination of yellowing is a particularadvantage for adhesives which are to be used in locations whereappearance is important, e.g. in automobile manufacture. Amines may alsogive rise to odour problems.

The invention provides a polymerisable adhesive composition for bondinga low surface energy substrate to a similar or different substrate,which includes:

-   -   a) at least one free-radically polymerisable monomer component,        and    -   b) an effective amount of an initiator system for initiating        polymerisation of the free-radically polymerisable monomer, said        initiator system comprising a compound of the formula I as        defined above,        with the proviso that the composition does not contain a        light-absorbing dye.

The polymerisable composition may suitably be a two-part composition inwhich at least one free-radically polymerisable monomer component isprovided in one part and the initiator compound of formula I is providedin the other part. Alternatively the composition could be a one-partcomposition with suitable stabilisation and activation systems such as amoisture-latent acid or oxygen-latent acid. In a further alternative,the initiator compound of formula I could be provided as a primer whichis applied to a substrate separate from the polymerisable monomercomponent.

The polymerisable monomer or monomers may suitably be one or more(meth)acrylic monomers.

In one aspect the invention provides a two-part polymerisable adhesivecomposition for bonding a low surface energy substrate to a similar ordifferent substrate, which includes:

-   -   part A) an effective amount of a polymerisation initiator        comprising a compound of the formula I as defined above, and a        carrier which is inert to the compound of formula I;    -   part B) at least one (meth)acrylate monomer, optionally with a        toughener, acidic monomer, filler or thickener.

Part B may suitably be a (meth)acrylic component. The terms(meth)acrylic and (meth)acrylate are used synonymously herein withregard to the monomer and monomer-containing component. The terms(meth)acrylic and (meth)acrylate include acrylic, methacrylic, acrylateand methacrylate.

No cationic dyes, visible light or ultraviolet radical polymerisationinitiators or hexaaryldiimidazole are required in the adhesivecompositions of the invention

The adhesive compositions disclosed herein are useful for bonding lowsurface energy substrates e.g. those having a surface energy of lessthan 45 mJ/m², more particularly polyolefins including polyethylene andpolypropylene, acrylonitrile-butadiene-styrene andpolytetrafluroethylene, or relatively low surface energy substrates suchas polycarbonate, to similar substrates, to each other, or to differentsubstrates including metals, other plastics and glass.

Furthermore the invention provides a method for bonding a low surfaceenergy substrate to a similar or different substrate, wherein the methodcomprises applying an adhesive composition as defined above to at leastone of the substrates, bringing the substrates together and allowing thecomposition to cure. The curing step does not involvephotopolymerisation. In one aspect, the invention provides a method asdefined above for bonding polyolefin substrates.

In particular the invention provides a method for bonding a low surfaceenergy substrate to a similar or different substrate, wherein the methodcomprises mixing parts A and B of a two part adhesive compositionimmediately prior to use in order to initiate polymerisation, applyingthe mixed adhesive composition to at least one of the substrates,bringing the substrates together and allowing the composition to cure bycompletion of the polymerisation initiated on mixing of the two parts Aand B

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the invention M is an alkali metal ion suchas lithium, sodium, potassium, or caesium, particularly lithium, sodiumor potassium, although metallic elements in the second row of thePeriodic Table such as barium, magnesium or calcium may be useable, asmay transition metals such as copper, iron or cobalt.

R¹–R⁴ may suitably be the same or different alkyl groups and each maysuitably be a C₁–C₆ alkyl group, particularly a C₂–C₄ alkyl group. Largeor bulky alkyl groups are considered to be less likely to providedesirably active initiators, as will be understood by those skilled infree-radical chemistry.

An example of a compound in which R¹–R⁴ are the same alkyl group isSodium tetraethylborohydride, alternatively called sodiumtetraethylborate, which is commercially available from various supplierssuch as Sigma-Aldrich Ireland Limited. Other suitable compounds includelithium tetraethyl borohydride, alternatively called lithiumtetraethylborate, lithium phenyl triethyl borate and tetramethylammoniumphenyl triethyl borate.

Desirably at least two, particularly all three, of R², R³ and R⁴ areC₁–C₁₀ alkyl. Suitably one or more of R², R³ and R⁴ may be phenyl.Desirably not more than one of R², R³ and R⁴ contains a phenyl group. Itwill be understood by those skilled in the art that a phenyl group (ifpresent) may be substituted in the ring by one or more substituentswhich do not affect the activity of the compound of formula I as apolymerisation initiator. Such ring-substituents include C₁–C₁₀ alkyl,for example C₁–C₆ alkyl, particularly methyl.

An initiator compound which is not a solid may suitably be used in asolvent such as Tetrahydrofuran, diglyme, dibutyl ether, toluene or ahydrocarbon solvent, suitably at a 1 molar concentration preferably notgreated thanabout 1.5 molar, although the person skilled in the artwillselect a suitable concentration depending upon the solvent used.

The quantity of initiator compound of formula I may suitably be such asto provide 0.01% to 5% by weight, particularly 0.01% to 2% by weight,such as 0.01 to 0.6% by weight, of boron in the total composition. Forease in handling of an initiator compound which is not used as a solid,the initiator compound is suitably used in a solution in a concentrationup to about 1.5 M, such as about 1 M.

The free-radically polymerisable monomer may be selected fromolefinically unsaturated systems such as acrylates, methacrylates,styrene, maleate esters, fumarate esters, unsaturated polyester resins,alkyd resins, thiol-ene compositions, and acrylate, methacrylate, orvinyl terminated resins including silicones and urethanes. Amongsuitable acrylates and methacrylates are those used in polymerisablesystems such as disclosed in U.S. Pat. No. 4,295,909, U.S. Pat. No.4,018,851, U.S. Pat. No. 4,963,220 to Baccei et. al., and U.S. Pat. No.4,215,209 to Ray-Chaudhuri et. al. or polyfunctional methacrylatessilicone diacrylates and polyfunctional acrylated urethanes of the typeknown to be useful in formulating adhesives e.g. as disclosed in U.S.Pat. No. 4,092,376 to Douek et. al.) or a thiol-ene (e.g. as disclosedin U.S. Pat. Nos. 3,661,744, 3,898,349, 4,008,341 or 4,808,638). Thecontents of the above-mentioned patents are incorporated herein byreference.

Suitable monomers include monofunctional acrylate and methacrylateesters and substituted derivatives thereof such as hydroxy, amide,cyano, chloro, and silane derivatives Such monomers includetetrahydrofurfuryl (meth)acrylate, methyl (meth)acrylate, ethyl(meth)acrylate, isobornyl methacrylate, hydroxyethyl methacrylate,hydroxypropyl (meth)acrylate, butyl acrylate, n-octyl acrylate,2-ethylhexyl (meth)acrylate, decylmethacrylate, dodecyl methacrylate,cyclohexyl methacrylate, tert.-butyl methacrylate, acrylamide,gamma-methacryloxypropyl trimethoxysilane, 2-cyanoethyl acrylate,3-cyanopropyl acrylate, tetrahydrofurfuryl chloroacrylate, and glycidyl(meth)acrylate.

Blends of two or more monomers, particularly two or more (meth)acrylatemonomers, may desirably be used, the choice of monomers in the blendbeing determined by the end use application of the compositions, asknown to those skilled in the art.

The addition of an acid (which term includes a latent acid) in themonomer component (Part B) is desirable. For certain substrates and/orcertain polymerisation speeds, the presence of an acid may be required.In other cases, acidic residues, for example in the monomers, may besufficient. Alternatively, or in addition, the initiator compound offormula I may react with another compound in the composition(particularly in Part B of a two-part composition), thus freeing thealkylborane to initiate polymerisation of the polymerisable composition.In the case of a 2-part composition, the reaction takes place aftermixing of the two parts, one containing the initiator compound offormula I and the other containing the compound reactive therewith.

The acid when added may suitably be a weak acid. Lewis acids may be used(see U.S. Pat. No. 5,539,070 to Zharov et al., the contents of which areincorporated herein by reference). The pKa of the weak acid normally isno lower than about 0.5 with the desirable limit being about 0.9. Theupper limit normally is about 13, or less, such as 11.5. However,carboxylic acids which have a pKa of up to about 8, such as 6 or 7, areparticularly suitable.

The carboxylic acids may contain one or more carboxyl groups, suitably 1to 4, and more preferably 1 to 2, carboxyl groups. Suitable aliphaticcarboxylic acids ordinarily include C₁₋₁₈ chains, such as C₁₋₁₀monocarboxylic acids.

Suitable acids may be monobasic or polybasic. Typical but not limitingexamples of suitable acids are formic acid, acetic acid, propionic acid,maleic acid, malic acid, fumaric acid, acrylic acid and copolymersthereof, methacrylic acid and copolymers thereof, pyruvic acid, itaconicacid, nadic acid, benzoic acid, phthalic acids, cinnamic acid,trichloroacetic acid and saccharin. It is particularly suitable to usean acidic monomer which can itself be polymerised, so that it is boundinto the cured polymer composition, for example a part-ester of apolyfunctional acid wherein the ester group contains a free-radicallypolymerisable component, particularly a (meth)acrylic halfester of adifunctional acid such as maleic, fumaric or succinnic acid e.g.2-(meth)acryloyloxyethyl maleate, 2-(meth)acryloxyethyl fumarate or2-(meth) acryloxyethyl succinate. The effective amount of the acid (orthe acid residue of an acidic monomer or residue resulting frompreparation of a monomer) is suitably within the range from about 0.1 toabout 20%, particularly from about 0.1 to about 10%, such as from about0.5 to about 5%, by weight based on the weight of the polymerisablecomposition. The amount of an acidic monomer is suitably within therange from about 0.1% to about 25%, such as about 0.5% to about 15%, byweight based on the weight of the polymerisable composition.

The acid may be present as a latent acid, particularly a maskedcarboxylic acid compound hydrolyzable on contact with moisture, such asan acid anhydride, as described in EP-A-0 356 875 and U.S. Pat. No.5,268, 436, the contents of which are incorporated herein by reference.A latent acid may be used in Part B of a two-part composition, in whichthe acid is released on mixing the two parts, or in a one-partcomposition.

The presence of air or oxygen is desirable at the time of initiation ofpolymerisation e.g. when the two parts of a two-part composition aremixed.

The initiator compound of formula I is desirably used with a carrierwhich is non-reactive with the initiator compound. Most suitably, thecarrier should be liquid, should be capable of supporting and carryingthe initiator compound, should be moisture-free, non-reactive with abase and not susceptible to free-radical polymerisation. The carrier maysuitably be a solvent for the initiator compound.

It is important that the initiator compound and carrier should not reacttogether or start to cross-link, causing a change in viscosity, beforepolymerisation of the total polymerisable composition is initiated.Suitable carriers include aziridine-functional materials which aredescribed as diluents in PCT Publication No. WO 99/64528, which in turnrefers to PCT Publication No., WO 98/17694, equivalent to U.S. Pat. No.5,935,711, the contents of all of which are incorporated herein byreference. When used, the initiator compound of formula 1 is carried by(e.g., dissolved in or diluted by) an aziridine-functional material or ablend of two or more different aziridine-functional materials in theinitiator component. Generally, the aziridine-functional material shouldnot be reactive toward the initiator compound and functions as anextender for the initiator. Also advantageously, theaziridine-functional material may generally increase the spontaneouscombustion temperature of the initiator component.

An “aziridine-functional material” refers to an organic compound havingat least one aziridine ring or group,

the carbon atom(s) of which may optionally be substituted by C₁–C₁₀alkyl groups, particularly C₁–C₃ alkyl groups.

Suitable aziridine-functional materials are described in WO 98/17694(U.S. Pat. No. 5,935,711). Polydifunctional aziridines such astrimethylolpropane tris(3-(2-methylaziridino) propionate areparticularly suitable. The aziridine-functional material may also act asa cross-linking agent in the polymerisable composition, e.g. by reactionwith an acid monomer or polymeric acid which may suitably be present inthe composition as described above.

The aziridine-functional material should be generally soluble inmonomers included in the polymerisable composition, such that the partsof the two-part composition can be readily mixed. By “soluble” is meantthat no evidence of gross phase separation at room temperature (i.e.,about 22° C. to about 25° C.) is visible to the unaided eye. Similarly,the initiator compound of formula 1 should also desirably be soluble inthe aziridine-functional material, although slightly warming a mixtureof the initiator compound and the aziridine-functional material may behelpful in forming a solution of the two at room temperature (i.e.,about 22° C. to about 25° C.). Accordingly, preferably, if used, theaziridine-functional material is a liquid at or near room temperature(i.e., within about 10° C. of room temperature) or forms a liquidsolution with the initiator compound at or near room temperature.

Other suitable carriers include liquid polyethers, liquid polyetherscapped with non-reactive groups being groups which are not susceptibleto free-radical polymerisation such as epoxies, liquid polyesters,polyisoprene or polybutadiene. A thickened solvent could also be used asa carrier. Polytetrahydrofuran could be used as both solvent andcarrier. The quantity of carrier may suitably be in the range from about5% to about 50% by weight, for example 5% to 25% by weight, particularly5% to 10% by weight, of the total composition. It is an advantage of thepresent invention that a wider range of carriers can be used than withan amine-containing initiator system, e.g. an epoxy which could reactwith the amine.

The initiator compound of formula I and the carrier, optionally with athickener or filler, are usually contained in Part A—the initiatorcomponent—of a two-part composition. If desired, Part A may also containa complexing agent or sequestering agent, e.g. a calixarene or apolyether or polythioether, e.g. a crown ether or thiocrown etherrespectively, with affinity for the metal in the initiator compound.

The monomer containing component—Part B of a two-part composition—maysuitably include a toughener to improve the impact resistance and peelresistance of the bond while maintaining adhesive strength. Suitabletougheners include elastomeric materials such as polybutadiene rubbers,polyisoprene (.e.g. available under the trade name Kratan),acrylonitrile-butadiene-styrene (e.g. available under the trade nameHycaror or as core-shell polymers under the trade name Blendex), orpolystyrenes. For bonding polyolefins it is desirable to use acore-shell polymer. The use of core-shell polymers in(meth)acrylate-based compositions is described, for example, in U.S.Pat. Nos. 4,536,546 and 4,942,201 Briggs et al. assigned to IllinoisTool Works. Core shell polymers are suitably graft copolymer resins(e.g. acrylonitrile-butadiene-styrene graft copolymers or othersdescribed in the above-mentioned patents of Briggs et al.) in the formof particles that comprise rubber or rubber—like cores or networks thatare surrounded by relatively hard shells. In addition to improving theimpact resistance of the bond, core-shell polymers can also impartenhanced spreading and flow properties to the composition (see WO99/64528). These enhanced properties include a reduced tendency for thecomposition to leave an undesirable “string” upon dispensing from asyringe-type applicator, or sap or slump after having been applied to avertical surface. The quantity of toughener may suitably be in the rangefrom about 1% to about 40% by weight, particularly about 5% to about 25%by weight, of the total composition.

The composition may optionally further comprise oxidising agents,reducing agents, thickeners, fillers, non-reactive colourants andpigments, metal salts (particularly transition metal salts) and freeradical polymerisation stabilisers. The optional additives are used inan amount that does not significantly adversely affect thepolymerisation process or the desired properties of polymerisationproducts made thereby. Ingredients for a photopolymerisation system arenot required and may be excluded.

Suitable reducing agents may be, but are not limited to,acetylphenylhydrazine, tetramethylthiourea or thiocaprolactam. Suitableoxidising agents may be, but are not limited to, peroxides andhydroperoxides.

Polymeric thickeners may be present in the compositions in a minoramount, up to about 50%, and may be thickeners such as a polymer orprepolymer of low or high molecular weight. Suitable polymericthickeners are a commercially available methacrylate polymer sold byE.I. du Pont de Nemours and Company, under the trademark Elvacite, aswell as styrene-methyl methacrylate co-polymers and polybisphenol Amaleate or propoxylated bisphenol-A-fumarate polyester (sold under thetrademark Atlac). It is also possible to add inert filling materialssuch as finely divided silica, fumed silica (treated or untreated),montmorillonite, clay, bentonite and the like. The use of micronizedsilica would result in a paste-like thixotropic composition.

It is an advantage of the present invention that a broader range ofthickeners can be used than with an initiator system containing amine.Additionally, it is conventional to include in adhesive formulationscertain “inert” fillers such as wood flour, glass fibres and hollowglass spheres, cotton linters, mica, alumina, silica and the like tomodify viscosity, improve impact resistance and for other purposes suchas spacing to provide a minimum bond thickness in the case of hollowglass spheres, e.g. at least 100 micrometres. Such fillers could beincorporated in the formulations of the present invention. The quantityof filler is suitably from about 0.5% to about 20%, for example about1.0% to about5.0%, by weight of the composition. Small percentages ofsilane monomers could also be added to increase moisture resistance, aswell as to enhance bonding of an adhesive to glass and similar surfaces.Other substances such as dyes, fire retarders, stabilizers such asquinones and hydroquinones, thixotropes, plasticizers, antioxidants, andthe like may also be included, although such additives may often befurnished in the principal ingredients, making their separateintroduction unnecessary.

It is a particular advantage of the invention that polyolefin materialscan be used as fillers in the composition. Polyolefin powders such aspolyethylene powder or polypropylene powder are relatively inexpensiveand in a composition which readily bonds to a polyolefin the fillerbecomes securely adhered into the cured composition. Polyethylene orpolypropylene powders can be used with particle sizes in the range from0.01 mm to 1 mm, particularly 0.02 mm to 0.3 mm, allowing a good controlof the gap between substrates (i.e. the depth of adhesive). Particularlysuitable polyethylene powders are commercially available under the tradename Microthene. Polyethylene flock and polyolefin chopped fibre canalso be used as fillers. The amount of polyolefin filler may suitably be0.5% to about 20%, particularly 1.0% to about 10%, by weight of thecomposition.

Compositions of the present invention may be used as adhesives,sealants, surface coatings, moulding resins and composite matrices, forexample with “fleece” or “padding” materials of glass fibre, carbonfibre, metal fibre, polyethylene or polypropylene fibre or foams, or anycombination of them, in which bonding to a low surface energy polymer isrequired.

The compositions may be used in an un-polymerised state, in which casepolymerisation occurs in situ, or they may be used in a part-polymerisedstate, in which case polymerisation is completed in situ.

The components of a two-part composition may suitably be mixedimmediately prior to use in a manner known to those skilled in the art.

The compositions of the invention are suited for use with conventional,commercially available dispensing equipment for two-part adhesives, forexample a dual syringe applicator and a static mixer nozzle. In generalthe compositions are suitable for curing at room temperature, i.e.20–25° C. without added heat or other energy input although heat inputmay be desirable in some instances, for example to accelerate cure.

The compositions of the present invention may suitably comprise about0.01% to about 5% by weight of the initiator, about 5% to about 50% byweight of the carrier (if present), about 0.1% to about 20% by weight ofthe acid (if present), about 5% to about 85% by weight of thepolymerisable monomer(s) and about 1% to about 40% by weight of thetoughner (if present).

The compositions of the invention may suitably consist essentially ofthe ingredients defined in the preceding sentence, together withconventional ingredients such as fillers, thickeners or stabilizers,making up the total composition as 100%.

In a two-part composition, the parts may suitably be provided in aweight ratio in the range from 1:10 to 1:1 of initiator Component A tomonomer-containing Component B, for example from 1:10 to 1:2, or from1:10 to 1:4.

EXAMPLES

The following examples will illustrate the invention. In the followingexamples the sodium tetraethyl borate and the trimethylolpropanetris(3-(2-methylaziridino) propionate used are commercially availablefrom Sigma-Aldrich Ireland Limited of Tallaght, Dublin 24, Ireland. Thecore-shell tougheners used are commercially available from GE SpecialityChemicals via Blagden Chemical Specialities Limited, London WC1X 8NJ,England under the tradenames Blendex 336 and Blendex 360. The fumedsilica used is commercially available under the trade name Aerosil R972from Degussa via Philips Duphar (Ireland) Limited, Dublin, Ireland.

In the examples the mixed adhesive composition is prepared using aMIXPAC System 50 1:10 volume ratio dual syringe applicator and a 17stage static mixer nozzle, both commercially available from METIX (UK)Limited of Kettering NN16 8PX, England.

The tests on polypropylene substrates were carried out on filledpolypropylene (i.e. polypropylene PP-HWST supplied by Simona U.K.(Limited) of Stafford, England) unless otherwise indicated.

In carrying out the tests, no tendency to catch fire was observed duringthe handling of the initiator compounds

Example 1

Initiator Component (Part A)

Sodium tetraethyl borate (1 g) was mixed with trimethylolpropanetris(3-(2-methylaziridino) propionate thickened with 5% by weight offumed silica available commercially under the trade name Aerosil R972from Degussa (25 g) and dissolved by stirring for approximately twohours.

Adhesive Component (Part B)

A slurry comprising a core-shell toughner (100 g), availablecommercially under the tradename Blendex 360 from GE SpecialityChemicals, tetrahydrofuriuryl methacrylate (249.5 g), 2-ethylhexylmethacrylate (83.2 g), 2-acryloyloxyethyl maleate (67.3 g) and glassspheres available commercially from Aldrich was stirred with a highshear mixer for several hours until a homogeneous dispersion wasobtained.

Adhesive

The initiator component A (5 g) and adhesive component B (45 g) werepackaged respectively into the two syringes of a MIXPAC System 50 1:10volume ratio 50 ml dual syringe applicator having a 1:10 volume ratiobetween the syringes holding the components A and B respectively. Thetwo components were mixed by the simultaneous extrusion through a 17stage static mixer nozzle. The mixed adhesive composition was tested ona range of substrates as follows:

Samples of the adhesive were spread onto an untreated test-piece (4×1inches)-101.6×25.4 mm) to form a film of approximately 0.1 mm in depth.Following which a second test-piece was brought against the adhesive toform an overlap adhesive joint with 0.25 inch (6.3 mm) overlap. A clampwas applied to the overlap area and the adhesive joint allowed to cureovernight, approximately 24 hours, at room temperature. The bondstrengths for bonds assembled using the above procedure were tested on aInstron tester according to ASTM-D1002 and are presented in the tablebelow.

Shear Strengths Substrates MPa Polyethylene/Polyethylene 2.9Polypropylene/Polypropylene 3.0 Mild Steel/Polyethylene 4.6 MildSteel/Polypropylene 5.0 Mild Steel/Mild Steel 13

Example 2

Initiator Component (Part A)

Lithium phenyl triethyl borate was prepared¹ by reacting 10 mls triethylborane (1 Molar in THF) in 30 mls of N-Hexane with 5.6 mls of phenyllithium (1.8M in cyclohexane/ether) added dropwise over a ten minuteperiod under a nitrogen atmosphere at −85° C. to −90° C. The resultingsolution was stirred mechanically for 15 minutes at −85° C. to −90° C.and allowed to return to room temperature and stirred for a furtherhour. The solvents were removed and trimethylolpropanetris(3-(2-methylaziridino)) propionate thickened with 5% by weight offumed silica Aerosil R972 manufactured by Degussa (3 g).

1. J. Polymer Science Part A Polym. Chem. 1996,34,13 p.

Adhesive Component (Part B)

The adhesive component was prepared as in Example 1.

Adhesive

A mixed adhesive composition was prepared as in Example 1. The mixedadhesive composition was tested on a range of subtrates as outlined inExample 1. The bond strengths are presented in the table below.

Shear Strengths Substrates MPa Polyethylene/Polyethylene 3.6Polypropylene/Polypropylene 2.3 Mild Steel/Polyethylene 3.4 MildSteel/Polypropylene 2.8 Mild Steel/Mild Steel 9.0

Example 3 (Comparative)

Initiator Component (Part A)

An organic borate photoinitiator CGI 1584 from Ciba, Basle, Switzerland,(0.2 g) was added to trimethylolpropanetris(3-(2-methylaziridino))propionate (5 g) and stirred until itdissolved.

Adhesive Component (Part B)

A slurry comprising a core-shell toughener (100 g), availablecommercially under the tradename Blendex 336 from GE SpecialtyChemicals, tetrahydrofurfryl methacrylate (249.5 g), 2-ethylhexylmethacrylate (83.2 g) and 2-acroyloyloxyethyl maleate (67.3 g) wasstirred with a high shear mixer for several hours until a homogeneousdispersion was obtained.

Adhesive

The mixed adhesive composition was prepared as in Example 1. The mixedadhesive composition was tested on a range of substrates as outlined inExample 1. The bond strengths are presented in the table below.

Shear Strengths Substrates MPa Polyethylene/Polyethylene 0Polypropylene/Polypropylene 0 Mild Steel/Polyethylene 0 MildSteel/Polypropylene 0 Mild Steel/Mild Steel 0

A similar experiment was carried out with the Ciba Photoinitiator CGI813, tetra methyl ammonium borate, again giving the same result i.e. nobond strengths on either polyolefins or mild steel.

Example 4

Initiator Component (Part A)

Tetra methyl ammonium phenyl triethyl borate (0.5 g) prepared¹ asoutlined in the literature was added to trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 5% by weight offumed silica Aerosil R972 manufactured by Degussa (4.5 g).

1. J. Polymer Science Part A Polym. Chem. 1996,34,13 p

Adhesive Component (Part B)

A slurry comprising a core-shell toughener (100 g), availablecommercially under the tradename Blendex 360 from GE SpecialtyChemicals, tetrahydrofurfuryl methacrylate (249.5 g), 2-ethylhexylmethacrylate (83.2 g) and 2-acryloyloxyethyl maleate (67.3 g) wasstirred with a high shear mixer for several hours until a homogeneousdispersion was obtained.

Adhesive

The mixed adhesive composition was prepared as in Example 1. The mixedadhesive composition was tested on a range of substrates as outlined inExample 1. The bond strengths are presented in the table below.

Shear Strengths Substrates MPa Polyethylene/Polyethylene 2.3Polypropylene/Polypropylene 2.6 Mild Steel/Polyethylene 3.0 MildSteel/Polypropylene 3.0 Mild Steel/Mild Steel 8.0

Example 5

Initiator Component (Part A)

The initiator component is prepared as outlined in Example 1.

Adhesive Component (Part B)

A slurry having the ingredients of Adhesive Component B of Example 1together with a micronised polyethylene powder (50 g) commerciallyavailable under the tradename Microthene FN500 distributed by NationalChemical Company, Ireland, is stirred with a high shear mixer forseveral hours until a homogeneous dispersion is obtained.

Adhesive

A mixed adhesive composition is prepared as outlined in Example 1.

1. A polymerisable adhesive composition for bonding a low surface energysubstrate to a similar or different substrate, comprising: a) at leastone free-radically polymerisable monomer component, b) an effectiveamount of an initiator system for initiating polymerisation of thefree-radically polymerisable monomer, wherein said initiator systemcomprises a compound of the formula I:

 wherein R¹ is C₁–C₁₀ alkyl, R², R³ and R⁴, which may be the same ordifferent, are selected from the group consisting of C₁–C₁₀ alkyl orC₃–C₁₀ cycloalkyl, phenyl, and phenyl-substituted C₁–C₁₀ alkyl or C₃–C₁₀cycloalkyl, provided that any two of R¹–R⁴ may optionally be part of acarbocyclic ring, and M⁺ is a metal ion, or a quaternary ammoniumcation, and c) an aziridine-functional material.
 2. A compositionaccording to claim 1 which is a two-part composition in which thefree-radically polymerisable monomer component is provided in one partand the initiator compound of the formula I is provided in the otherpart.
 3. A two-part polymerisable adhesive composition according toclaim 1 comprising: part A) an effective amount of a polymerisationinitiator comprising a compound of the formula I as defined above, and acarrier which is inert to the compound of formula I; part B) at leastone (meth)acrylate monomer, optionally with a toughener, acidic monomer,filler or thickener.
 4. A composition according to claim 1, wherein theinitiator compound is of formula I in which each of R¹–R⁴ is a C₂–C₆alkyl group.
 5. A composition according to claim 1, wherein theinitiator compound is of formula I in which each of R¹R⁴ is a C₂–C₄alkyl group.
 6. A composition according to claim 1, wherein theinitiator compound is of formula I in which M+ is an alkali metal ion.7. A composition according to claim 1 wherein the initiator compound isof formula I in which M+ is a quaternary ammonium cation of the formulaII:

wherein R⁵–R⁸, which may be the same or different, are C₁–C₁₀ alkyl,C₁–C₁₀ alkenyl, C₃–C₁₀ cycloalkyl, aryl, C₁–C₁₀ alkylaryl, arylC₁–C₁₀alkyl or arylC₁–C₁₀ cycloalkyl, provided that any two of R⁵–R⁸ may bepart of an optionally unsaturated carbocyclic ring.
 8. A compositionaccording to claim 6 wherein the alkali metal salt is selected fromsodium tetraethylborate and lithium tetraethylborate.
 9. A compositionaccording to claim 1 wherein the initiator compound is of formula I inwhich three of R¹–R⁴ are the same C₁–C₁₀ alkyl group and one of R¹–R⁴ isphenyl or phenyl-substituted C₁–C₁₀ alkyl.
 10. A composition accordingto claim 1 wherein the initiator compound is of formula I in which threeof R¹–R⁴ are the same C₂–C₄ alkyl group and one of R¹–R⁴ is phenyl. 11.A composition according to claim 9 wherein the initiator compound isselected from the group consisting of lithium phenyl triethyl borate andsodium phenyl triethyl borate.
 12. A composition according to claim 1,further comprising a polyolefin filler.
 13. A composition according toclaim 1, wherein the free-radically polymerisable monomer componentcomprises a (meth)acrylic monomer.
 14. A composition according to claim12, wherein the free-radically polymerisable monomer component comprisesat least two (meth)acrylic monomers.
 15. A composition according toclaim 1, wherein the quantity of the initiator compound of formula I issuch as to provide 0.01% to 5% by weight, particularly 0.01% to 2% byweight of boron in the total composition.
 16. A composition according toclaim 1, which includes an acidic monomer which is free-radicallypolymerisable.
 17. A method for bonding a low surface energy substrateto a similar or different substrate, wherein the method comprisesapplying a composition according to claim 1 to at least one of thesubstrates, bringing the substrates together with the compositionbetween them and allowing the composition to cure.
 18. A method forbonding a low surface energy substrate to a similar or differentsubstrate, wherein the method comprises mixing parts A and B of a twopart adhesive composition according to claim 2 immediately prior to usein order to initiate polymerisation, applying the mixed adhesivecomposition to at least one of the substrates, bringing the substratestogether and allowing the composition to cure by completion of thepolymerisation initiated on mixing of the two parts A and B.
 19. A curedadhesive composition comprising the polymerisation product of acomposition according to claim
 1. 20. A bonded article comprising twosubstrates bonded by the polymerisation product of a compositionaccording to claim
 1. 21. A bond formed between two substrates by thepolymerisation product of a composition according to claim
 1. 22. Acomposition, according to claim 1, further comprising d) an effectiveamount of a compound that is reactive with the compound of formula I tofree an organoborane from the compound of formula I and thus to initiatepolymerisation of the at least one free-radically polymerisable monomer.23. A system capable of initiating the polymerisation of a (meth)acrylicmonomer for bonding a low surface energy substrate to a similar ordifferent substrate, the system comprising: a) a composition accordingto claim 1; and b) an effective amount of a compound that is reactivewith the compound of formula I to free an organoborane from the compoundof formula I.
 24. A composite article comprising a first substrate and asecond substrate bonded to the first substrate by an acrylic adhesive,wherein at least one of the first or second substrates is a low surfaceenergy substrate and wherein the acrylic adhesive comprises thepolymerisation product of a polymerisable acrylic composition thatcomprised: a) a composition according to claim 1, and b) an effectiveamount of a compound that is reactive with the compound of formula I tofree an organoborane from the compound of formula I and thus to initiatepolymerisation of the at least one (meth)acrylic monomer.
 25. A methodof bonding a low surface energy polymer to a substrate, the methodcomprising the steps of: i) providing a low surface energy polymer; ii)providing a substrate; iii) providing an adhesive composition accordingto claim 22; iv) applying the adhesive composition to either the lowsurface energy polymer or the substrate; v) joining the low surfaceenergy polymer and the substrate with the adhesive compositiontherebetween; and vi) permitting the adhesive composition to cure toadhesively bond the low surface energy polymer and the substrate.